#include <bits/stdc++.h>
#define FOR(i,a,b) for(int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define sz(n) (int)(n).size()
using namespace std;
typedef long long ll;

int main() {
	int n, a[101];
	cin >> n;
	int ans = 0;
	REP(i, n) cin >> a[i];
	FOR(i, 2, n) {
		if (a[i - 2] != a[i] && (a[i - 2] < a[i - 1] && a[i - 1] > a[i] || a[i - 2] > a[i - 1] && a[i - 1] < a[i])) ans++;
	}
	cout << ans << endl;
	return 0;
}